Obama condemns 'despicable' Dallas attacks

US President Barack Obama has denounced the "despicable" attacks in Dallas in which snipers shot dead five police officers during a protest against police shootings of black men.

"There has been a vicious, calculated and despicable attack on law enforcement," Mr Obama said during a press conference at a NATO summit in Warsaw, adding that the officers were "targeted" by multiple suspects.

Snipers operating from rooftops in Dallas killed five police officers and wounded six more in a co-ordinated attack during one of several protests across the country against the killing of two black men by police this week.

Police described last night's ambush as carefully planned and executed and had taken three people into custody.

A fourth died from what Dallas-based media said was a self-inflicted gunshot after a standoff that extended into this morning.

That suspect had exchanged gunfire with police during the stand-off at a downtown garage and warned of placing bombs throughout the city. Police have yet to confirm his death.

The attack was one of the worst mass shootings of police in US history.

President Obama described the killings as tragedies saying "all of us as Americans should be troubled by these shootings, because these are not isolated incidents.

"They're symptomatic of a broader set of racial disparities that exist in our criminal justice system."

White House officials have spoken with Dallas Mayor Mike Rawlings about the shooting that turned the downtown of one of the largest US cities into a sprawling crime scene, unfolding along streets that house major corporations, restaurants and government offices.

Dallas Police Chief David Brown said the shooters, some in elevated positions, used sniper rifles to fire at the officers in what appeared to be a coordinated attack.

"(They were) working together with rifles, triangulating at elevated positions in different points in the downtown area where the march ended up going," Chief Brown told a news conference, adding a civilian was also wounded.

Dallas shooting: Sound of gunfire as police approach the scenehttps://t.co/B02dhUwj1D

— RTÉ News (@rtenews) July 8, 2016

Dallas shooting: An eye witness describes the moment he heard the gunfirehttps://t.co/oWbUjY1AHB

— RTÉ News (@rtenews) July 8, 2016

Police said a bomb squad unit was investigating a suspicious package found near one suspect's location following their arrest.

"Our worst nightmare has happened," Mayor Rawlings, told a news conference. "It is a heartbreaking moment for the city of Dallas."

It has been a devastating night. We are sad to report a fifth officer has died.

— Dallas Police Depart (@DallasPD) July 8, 2016

One of the officers killed has been identified as Brent Thompson, 43, an officer with Dallas Area Rapid Transit.

Mayor Rawlings advised people to stay away from downtown Dallas as police combed the area where large areas have been cordoned off and transport halted.

Federal authorities also halted commercial air traffic for the area as police helicopters hovered over the scene.

The Dallas-Fort Worth metropolitan area is one of the nation's most populous and is home to more than seven million people.

The shooting happened as largely peaceful protests unfolded around the US after the shooting of Philando Castile, 32, by police near St Paul, Minnesota, late on Wednesday.

The suspect in the Dallas standoff had told police "the end is coming" and that more police were going to be hurt and killed.

Chief Brown said the suspect also told police "there are bombs all over the place in this garage and downtown".

Police said they were questioning two occupants of a Mercedes they had pulled over after the vehicle sped off on a downtown street with a man who threw a camouflaged bag inside the back of the car.

A woman was also taken into custody near the garage where the standoff was taking place.

"We are leaving every motive on the table on why this happened and how this happened," Brown said.

Mayor Rawlings visited the wounded at Parkland hospital, the same hospital where President John F Kennedy was taken after he was shot in Dallas in November 1963.

Outside, officers stood in formation and saluted as bodies of the officers were about to be transported.